Achievement of higher productivity and lower cost in processing melt-fabricable resins requires high-rate extrusion of melt-fabricable resins. However, melt-fabricable resin compositions inevitably have a critical shear rate and, if the extrusion rate is higher than the critical shear rate, the surface of a product becomes roughened (this phenomenon is called melt fracture), causing a failure in providing favorable molded articles.
One example of methods that can solve such a problem to avoid melt fracture and achieve a higher extrusion rate to improve the extrudability is a method of molding a material at a higher molding temperature. However, high-temperature molding causes pyrolysis of melt-fabricable resins, which raises problems such as reduction in mechanical properties of molded articles and staining on molded articles. In addition, the melt-fabricable resins are caused to have a lower melt viscosity, and thus drip off or deform before being cooled and solidified. This impairs the dimensional accuracy of molded articles.
Patent Literature 1 discloses, as another method, a method of producing an extrudable composition including the step of simultaneously mixing i) a first fluoroelastomer that has a first Mooney viscosity ML(1+10) at 121° C. determined in conformity with ASTM D-1646 in an amount of 0.001 to 10 wt % based on the total weight of the extrudable composition, ii) a second fluoroelastomer having a second Mooney viscosity ML(1+10) at 121° C. determined in conformity with ASTM D-1646 in an amount of 0.001 to 10 wt % based on the total weight of the extrudable composition, and iii) a non-fluorinated melt-fabricable polymer, with a difference between the first and second Mooney viscosities of at least 15.
Patent Literature 2 discloses a method including the steps of: preparing a melt-fabricable polymer composition that contains a melt-fabricable thermoplastic host polymer and an effective amount of an additive composition for processing that contains a specific multimode fluoropolymer; mixing the additive composition for processing and the host polymer for a time enough for thorough mixing of them; and melt-fabricating the polymer composition.
The following documents disclose techniques using a fluoropolymer as a processing aid. Patent Literature 3 discloses an extrudable composition containing a thermoplastic hydrocarbon polymer, a poly(oxyalkylene) polymer, and a fluorocarbon polymer. Patent Literature 4 discloses an extrudable composition containing a resin blend that contains a metallocene linear low-density polyethylene resin and a low-density polyethylene resin, a fluoroelastomer that has a Mooney viscosity ML(1+10) at 121° C. of 30 to 60, and a surfactant. Patent Literature 5 discloses a processing aid containing a fluorine-containing polymer that has an acid value of 0.5 KOHmg/g or higher.
Patent Literature 6 discloses a processing aid to be mixed with a thermoplastic hydrocarbon polymer, containing a copolymer of a fluorinated olefin monomer and a substantially non-fluorinated hydrocarbon olefin monomer.